1. Field of the Invention
The present invention relates generally to dust collection using electrostatic precipitators in power generation plants, and more particularly to flue gas conditioning that adds trace amounts of sulfur trioxide (SO3) into the flue gas stream.
2. Description of the Prior Art
Prior art Flue Gas Conditioning (FGC) of fly ash involves addition of trace amounts of SO3 into flue gas stream to control an electrical resistivity of a dust and improve its collection in an electrostatic precipitator (ESP). FGC makes it possible to significantly improve precipitator collection efficiency at a cost considerably less than that of alternatives.
Typically, SO3 is formed by a catalytic conversion of gaseous sulfur dioxide (SO2). The SO2 usually comes from an “external” source (feedstock) by either evaporating liquid sulfur dioxide or by burning molten or solid sulfur. Conventional flue gas conditioning systems have two major drawbacks: they require an external continuous supply of a feedstock and in the process of operation these systems slightly increase SO2 emissions.
Another prior art approach to create SO3 is to utilize “native” SO2 formed during combustion of sulfur contained in fossil fuels as a feedstock for a subsequent conversion to SO3. Extensive experiments known in the art have confirmed that at the conditions typical for coal-fired applications substantial portion of the “native” SO2 could be oxidized to SO3. The rate of such process will depend on the temperature, the concentrations of SO2, O2 and water vapors as well as catalyst's properties. Systems implementing this technique have been proposed, but none are free of major deficiencies.
The use of additives to improve dust collection rates originated shortly after commercial use of ESP's begun early in the last century. Evaporation of acid in smelter gases to reduce dust resistivity occurred in 1912; water injection into gases from cement kilns and steel refining vessels has been used for many years, and ammonia treatment of catalyst dust in petroleum refineries begun in the 1940's. Early trials of FGC showed benefits which justified continued experimentation, and encounters with several dozens of ways of doing it wrong eventually developed a set of reliable rules for doing it right. Acceptance of the flue gas conditioning process as an engineering solution to a common environmental problem has come slowly, however, one result is that use of sulfur trioxide for adjustment of the resistivity of fly ash from low sulfur coal has been widely applied and has become an accepted part of the option of switching to low sulfur coal for compliance with the Clean Air Act of 1990. It was estimated that over 50,000 megawatts of generating capacity have utilized coal-switching plus flue gas conditioning for this purpose.
Since early 1970's most of the development and application of FGC has been devoted to improving collection of fly ash generated by boilers fired with pulverized coal in power generating stations. A large fraction of the world's coals have relatively low sulfur contents and tend to generate fly ashes having electrical resistivity too high for ESP collection at optimum rates. A typical relationship between ash resistivity and ESP collection rate for large high-efficiency precipitators is with collection being maximum in the range of resistivity about 1 to 5×109 ohm-cm, falling off rapidly as resistivity increases and somewhat more slowly as resistivity decreases.
Fly Ash Resistivity Vs. Precipitator Performance Relationship
The decrease in collection rate at high resistivity is due to a “back corona” phenomena (electrical breakdown in the layer of collected ash on the collecting plates) or to the necessary reduction of precipitator power input to avoid the onset of back corona. At lower resistivity the reduction is due to increases of rapping losses and ash reentrainment as electrical holding forces in the ash layer decrease. It can be seen that too high or too low resistivity can severely impact the ESP performance.
Electrical Resistivity of Fly Ash
Electrical resistively is one of the critical parameters influencing fly ash collection by electrostatic precipitators. The electrical resistivity of fly ash depends on the chemical composition of the ash, the constituents of flue gases, and the temperature. Fly ash composition is largely determined by the type and composition of the coal being burned, and the furnace operating conditions. At lower temperatures, fly ash resistivity is determined by electrical conduction over the surface of the particles. The latter is produced by the movement of ions in molecular thickness coatings on the particles, and is termed surface resistivity.
Surface conductivity is dependent on interaction between the flue gas and the ash. Environmental factors include temperature and the concentration of gaseous and condensed phases in contact with ash. Flue gas temperature influences the concentration of water vapor, the existence of a condensed phase, and the reactivity between the ash and environment. High temperature resistivity, when plotted against inverse absolute temperature, is a straight line, illustrating conduction through the bodies of the particles, called “bulk” or volume resistivity.
Resistivity Control
Sulfur occurs in coal as organic and inorganic compounds—notably pyrite and sulfate salts. When coal is burned, more than 95 percent of the sulfur appears in the flue gas in the form of sulfur dioxide. Thermodynamics is the process-limiting factor in the boiler, for the kinetics of oxidation does not allow more than a small fraction of the sulfur oxides to appear as sulfur trioxide. When the temperature of flue gas drops to around 300° C. (572° F.), a significant fraction of the sulfur trioxide gas reacts with water vapor to produce sulfuric acid vapor. This process is essentially complete at temperatures around 150° C. (302° F.) where electrostatic precipitators normally operate. The small fraction of the total of sulfur oxides occurring as sulfur trioxide at 149° C. (300° F.) can be sufficient to lower the resistivity of ash to an acceptable range. Sulfur trioxide and water vapor are jointly adsorbed or condensed on an otherwise poorly conducting surface. The sulfuric acid vapor adsorbed on the fly ash surface directly participates in the conduction process.
The possibility of using sulfuric acid to reduce excessively high dust resistivity was known and understood from very early experience with ESP's, but general application outside the non-ferrous metals industry did not occur until the early 1970's when three factors simultaneously contributed to increased acceptability. These were the enactment and enforcement of strict limits on the emission of particulate matter and sulfur products, concomitant increase in the use of low-sulfur coals, and development of automated FGC systems based on catalytic generation of sulfur trioxide.
Possible methods for producing of a “substitute” SO3 to be used for flue gas conditioning include evaporation of sulfuric acid or liquid sulfur trioxide, stripping of oleum, or catalytic conversion from sulfur dioxide. The last of these is the method chosen for all commercial installations in service at the present time because it minimizes the resident quantity of aggressively toxic material and it can be easily packaged and controlled. The system can be quickly purged, control is simple and automatic over the full range of boiler operation, it utilizes well-known and proven technology, and when burning of elemental sulfur is the source of the required sulfur dioxide, the feedstock cost is low. Liquid sulfur dioxide is sometimes used as the feedstock for small, temporary or short-lived systems, but permanent installations ordinarily use the sulfur burning process.
When resistivity control by these systems was first commercialized, the factors affecting the amount of sulfur trioxide to be injected for a given resistivity change were understood only in a general and rather superficial way. The obvious intent was to reproduce ash conditions which occurred when coal with sufficient sulfur content to produce acceptable ESP operation was burned, and on this basis rough estimates of injection rates could be made. However, early installations were intentionally made with a generous capacity margin in excess of the rough estimates and, although sufficient margin was present to handle the range of variation actually encountered, experience soon showed that the original estimates were by no means precise.
Determining Injection Rates
A parabolic characteristic of resistivity as a function of temperature in an exhaust flue gas is well known. Generally, resistivity has a maximum value at a temperature around 149° C. (300° F.) with decreasing values above and below the maximum point as illustrated by the above figure. In determining the amount of SO3 required to reduce the ash resistivity to a desired lower value, one would think that the maximum rate would be required at the maximum unconditioned resistivity point, with decreasing amounts at higher and lower temperatures. That is, a curve of injection rate reflecting the unconditioned resistivity characteristic would be expected.
An interesting discovery made in the development of SO3 flue gas conditioning is that the amount of SO3 required to attain a desired level of resistivity follows the expected dome-shaped curve with respect to temperature only up to a point, after which it breaks off to a rapidly rising characteristic as seen in the figure. The inflection point (a knee) between the two portions of the curve is a function of the surface chemistry of the ash, occurring at relatively low temperatures for acidic ashes and at higher temperatures for basic ashes.
The range of variation of the inflection point temperature appears to be approximately from 121 to 204° C. (250 to 400° F.) for coals available worldwide. If the flue gas temperature is above the inflection point, the portion of the injected SO3 which is greater than the level of the dome-shaped portion of the curve does not attach to the ash and will be passed through the ESP.
FIG. 7 shows a typical prior art sulfur burning FGC system.
Conventional or “External” Feedstock Sulfur Trioxide FGC
It is known in the art that almost all commercial SO3 FGC systems installed to date are based on catalytic conversion of SO2 to SO3. The SO2 is either supplied in liquid form or obtained by burning elemental sulfur.
Elemental sulfur is the preferred feedstock for long-term permanent operations because its operating costs are lower, but liquid SO2 is typically used for trials and small or short-term situations where reduced capital costs can offset increased feedstock expense. The catalytic conversion design was chosen over other methods by which SO3 may be made available partly because it is flexible and easily controllable, but mainly because it minimizes the quantity and the difficulty of handling hazardous materials resident in the system. The figure above is a diagrammatic representation of a typical sulfur-burning system for SO3 conditioning. Molten sulfur is delivered by thermally-insulated tank trucks fitted with steam coils for melt-out. In locations where sulfur cannot be delivered in molten form, bagged or bulk solid sulfur may be supplied for melting on site. The sulfur grade is designated as “Bright Yellow” which contains very low levels of contaminating materials. The exact analysis varies slightly among suppliers, but completely lacks chemicals which could act as catalyst poisons and contains only very minute quantities of hydrocarbons. Storage in insulated steel tanks with steam-blanketing provisions for fire suppression is standard, but concrete-lined pits are sometimes used. Tanks and molten sulfur piping are heated by steam controlled to a saturation temperature of approximately 143° C. (290° F.) at which the sulfur has ideal flow characteristics. Steam tracing is preferred because of the ease with which controllable highly uniform temperatures can be maintained throughout the system. Typically, sulfur metering pumps are supplied in duplicate so that one may be serviced while the other is in operation.
FIG. 8 below shows a prior art FGC system with a multi-pass converter.
Prior-Art FGC System with a Multi-Pass Converter.
Combustion of the sulfur to generate SO2 is obtained by introducing sulfur into an air stream which has been preheated for startup purposes to the temperature at which the catalyst becomes active. Since this temperature exceeds the auto-ignition point of molten sulfur, burning is initiated immediately upon the introduction of sulfur and SO2 is delivered to the catalyst for conversion to SO3. Roughly 4,000 Btu's of heat are generated per pound of sulfur burned. This replaces a portion of the startup heat input. Typically, at full system rating all the required heat to maintain the catalyst at operating temperature is supplied by sulfur combustion. Clearly, operation of the system in this manner allows the generation of any quantity of SO3 from zero to full system rating as a function of any selected control signal used to determine the rate at which sulfur is delivered.
In conventional FGC systems the catalyst for conversion of SO2 to SO3 usually is chosen from any of the types developed for the manufacture of sulfuric acid and similar applications. Vanadium pentoxide is the active ingredient in most of these, and is classified as a hazardous material. Some FGC system designers utilize so-called “multi-pass” SO2 to SO3 converter design shown in the figure above. Care must be taken to exclude dust and water from the air intake to the maximum extent possible, and to service the air intake filter on a regular basis.
The hot air stream containing the generated SO3 exits the SO2 to SO3 converter at temperatures from 399° C. (750° F.) to about 538° C. (1,000° F.), depending on the rate of SO3 production. It is essential that this stream be held above its acid dew point temperature all the way through the delivery manifold and injection probes. If the temperature goes below the dew point, the acid will condense out in the manifold or the injection probes. This is undesirable for two reasons. First, none of the acid will reach the flue gas to do the intended conditioning job, and, secondly, the condensed acid will corrode the piping and nozzles. Since the SO3 concentration is a few percent in this stream the acid dew point will be on the order of 238° C. (460° F.), but maintaining the delivery end of the system in a condensation-free state requires that the calculated gas temperature as it issues from the injection probe nozzles not be less than 260° C. (500° F.). For this reason the distribution manifolds are heavily insulated, and in addition the injection probes, if installed on the cold side of the air preheater, are thermally insulated from the flue gas.
The length of travel of the flue gas in the ductwork after the injection point required to provide essentially complete mixing with the SO3 is often referred as “one second mixing time” or “ten times the nozzle spacing”. These rules of thumb are derived from experiments showing that in turbulent flue gas flow complete mixing with another gas injected through a bank of nozzles arrayed as a uniformly spaced grid occurred at a distance downstream of the grid equal to about eight to ten times the nozzle spacing. In conventional FGC systems a nominal grid spacing of three feet has been found to be a reasonable compromise.
Because installation of injection probes is ordinarily more easily accomplished in the ESP-type ductwork on the cold side of the air pre-heater than in the boiler-type construction on the hot side, most of the present FGC installations inject on the cold side. It should be noted, however, that hot side installation has the advantages of lacking any close approach to acid condensation temperatures in the probes, and provides excellent mixing and contact between the SO3 and the fly ash as it passes through the air pre-heater. Probes for hot side installation are less expensive than cold side because no thermal insulation is required and the erosion-protective outer sheathing can usually be eliminated. As far as operation is concerned, every existing hot side installation works as well as or better than equivalent cold side units.
Native Feedstock FGC Technologies
Conventional sulfur trioxide injection systems work well, and are widely used. In some instances, however, there are drawbacks. The catalytic conversion of sulfur dioxide to sulfur trioxide is not completely efficient, and additional sulfur dioxide is added to the flue gas flow. A constant supply of sulfur feedstock is required, and this feedstock must be safely handled. All components of the burning, catalyzing, and injecting system must be kept in good working order, and there is a power consumption and O&M costs associated with the process.
As previously mentioned, another approach to create SO3 is to utilize a “native” SO2 formed during combustion of sulfur contained in fossil fuels as a feedstock for a subsequent conversion to SO3. Extensive experimentation at the Lehigh University was conducted to investigate a new approach to fly ash conditioning without an external addition of sulfur or sulfur dioxide. The method was based on causing the conversion to SO3 of a portion of the SO2 normally present in flue gas even when low sulfur coals were burned.
The thermodynamic laws of chemical equilibrium predict that when the gas contains about 5% O2, about 99% of the SO2 can be oxidized to SO3 at about 399° C. (750° F.) and about 90% at 510° C. (950° F.) with respectively more SO3 generated at lower temperatures and less at higher temperatures. In real life, however, not more than approximately 1-3% of the “natural” SO2 is being oxidized in SO3 with the rate of such reaction usually depending on the gas temperatures, the concentration of SO2, O2 and water vapors as well as possible catalytic properties of the boiler convection surfaces.
It has been shown in the art that it is possible to oxidize considerable quantities of SO2 present in flue gas at concentration typical for commercial coal-fired boilers burning low sulfur coal, by inserting commercial catalysts in the SO2-containing gas. Several systems implementing this technique have been subsequently proposed, but none are free of major deficiencies.